RAID questions/advise

[Michael S. Peek]

Hi guys,

I've had a couple of RAID boxes ticking away in the corner for years now 
without a problem.  But now our needs have expanded, and I'm looking to 
build replacements.  Big replacements.  And I consider myself to be 
anything but an expert in the field, especially where mdadm is 
concerned.  So I have a few questions to ask in hopes that someone out 
there can help me out.

How large of an array can mdadm handle?

If I use my hardware RAID cards in JBOD mode, how does the kernel handle 
naming drives when there's more than 26 drives on the system?  (i.e. 
what does it do when it reaches /dev/sdz and there are drives left to be 
named?)

 From what I hear, ext3 can handle filesystems up to 32TB in size, but 
has anyone actually done this?  Can anyone attest to how well it works?  
Or is there another filesystem type that's better suited to large (12TB 
- 32TB) filesystems?

Finally, according to the mdadm FAQ, when a drive goes down:
> 19. What should I do if a disk fails?
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   Replace it as soon as possible:
>     
>     mdadm --remove /dev/md0 /dev/sda1
>     halt
>     <replace disk and start the machine>
>     mdadm --add /dev/md0 /dev/sda1

Since my OS drive will not be a part of the RAID (it'll have a mirrored 
RAID of it's own), I presume that the halt command won't be necessary.  
I assume that it would be perfectly reasonable of me to remove the drive 
and replace it while the system is running?  I.e. mdadm can handle 
running in degraded mode for the duration of the replacement/rebuild 
process?  (This is a deal-breaker question -- if mdadm can't, then I'll 
have to pursue other measures.  Hotswap drives will be up and running at 
all times though, so I presume I can configure mdadm to make use of them 
immediately upon detecting a drive failure.)

My other option is to get a SAN/NAS of some type, but building machines 
like this has proven to be very effective and cheap compared to 
SAN/NAS'es, at least in the past.  I haven't checked recently though, 
but any advice is welcome.

Pondering my options,

Michael